Abstract: In view of pipe butt requirements of large diameter, thick wall and deep narrow gap groove in large engineering projects, a narrow gap TIG (NG-TIG) welding gun was designed to achieve high efficiency welding. The gun body was an asymmetrical integrated flat structure. The welding experiment was carried out in U groove with gap width of 10 mm and depth of 15 mm. It was found that there appeared some problems such as discontinuous weld forming, porosity and arc instability in narrow gap welding process. In order to achieve ideal effect of welding process, numerical simulation analysis was carried out by Fluent software in this paper to optimize structure of protective gas transport channel and outlet layout of narrow gap TIG welding gun. After being compared with traditional TIG welding gun in terms of protective gas speed and movement track, it was found that protective gas evenly diffused outward around tungsten electrode and its velocity was close to the gas protection state of traditional TIG welding gun, so as to determine gas protection structure of narrow gap TIG welding gun. The welding experiment showed that welding gun had characteristics of stable arc and no splash in the welding process under welding parameters of 120 A welding current, 100 mm/min welding speed and 25 L/min gas flow, and weld was well formed without defects.Highlights: (1) Fluent software was used to simulate protection gas state of traditional TIG welding gun, which was used as a reference to optimize air supply structure of narrow gap TIG welding gun.(2) Narrow gap TIG welding gun not only inherited advantages of traditional TIG, but also solved common technological problems in narrow gap welding process such as arc instability and weld porosity.(3) Through non-wire filling and wire filling, welding experiments were carried out under two working conditions of groove and pipe wall cover, and good applicability of narrow gap TIG welding gun was confirmed.